This invention pertains to methods for treating atrial tachyarrhythmias. In particular, the invention relates to an apparatus and method for delivering shock therapy to terminate atrial fibrillation.
Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). They can occur in either chamber of the heart (i.e., ventricles or atria) or both. Examples of tachyarrhythmias include sinus tachycardia, ventricular tachycardia, ventricular fibrillation (VF), atrial tachycardia, and atrial fibrillation (AF). Tachycardia is characterized by a rapid rate, either due to an ectopic excitatory focus or abnormal excitation by normal pacemaker tissue. Fibrillation occurs when the chamber depolarizes in a chaotic fashion with abnormal depolarization waveforms as reflected by an EKG.
An electrical shock applied to a heart chamber (i.e., defibrillation or cardioversion) can be used to terminate most tachyarrhythmias by depolarizing excitable myocardium, which thereby prolongs refractoriness, interrupts reentrant circuits, and discharges excitatory foci. Implantable cardioverter/defibrillators (ICDs) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device. An ICD is a computerized device containing a pulse generator that is usually implanted into the chest or abdominal wall. Electrodes connected by leads to the ICD are placed on the heart, or passed transvenously into the heart, to sense cardiac activity and to conduct the shock pulses from the pulse generator. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, and may also incorporate cardiac pacing functionality.
The most dangerous tachyarrythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. ICDs are also capable, however, of detecting atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter, and delivering a shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy.
As aforesaid, an ICD terminates atrial fibrillation by delivering a shock pulse to electrodes disposed in or near the atria. The resulting depolarization also spreads to the ventricles, however, and there is a risk that such an atrial shock pulse can actually induce ventricular fibrillation, a condition much worse than atrial fibrillation. To lessen this risk, current ICDs delay delivering an atrial shock pulse until the intrinsic ventricular rhythm is below a specified maximum rate and then deliver the shock synchronously with a sensed ventricular depolarization (i.e., an R wave). That is, a current R-R interval, which is the time between a presently sensed R wave and the preceding R wave, is measured. If the current R-R interval is above a specified minimum value, the interval is considered shockable and the atrial defibrillation shock pulse is delivered.
Judging a current R-R interval to be shockable or not based solely upon whether it exceeds a single specified minimum value, however, can lead to errors because the period during which the ventricle is vulnerable to fibrillation may not be reflected by the current R-R interval. For example, certain R-R interval sequences, such as a long-short R-R interval sequence, are particularly dangerous for shock timing which thus increases the risk of fibrillation for a given specified minimum interval. In order to lessen this risk, the specified minimum interval value can be increased, but this has the effect of delaying the delivery of atrial defibrillation therapy until the patient""s heart rate drops to a rate corresponding to the increased minimum interval value. It is an objective of the present invention to provide an improved method for detecting shockable R-R intervals so as to allow defibrillation shocks to be delivered in a safe and timely manner.
The present invention is a method and apparatus for delivering atrial defibrillation therapy in which delivery of an atrial defibrillation shock pulse is delivered synchronously with a sensed R-wave if the current R-R interval meets one or more safety criteria so as to be considered shockable. A first criterion defines a shockable R-R interval as one that exceeds the previous QT interval by a specified therapy margin. The previous QT interval may be determined by detecting a T-wave following an R-wave or estimated as a function of the measured preceding R-R interval. A second criterion may be applied that requires, in addition to meeting the first criterion, that a current R-R interval be longer than a specified minimum interval value in order to be considered shockable. A third criterion may also be applied which considers a current R-R interval shockable if it exceeds a specified sufficiently-long interval value irrespective of the length of the preceding R-R interval, where the sufficiently-long interval is longer than the specified minimum interval value.